The present invention relates to a cooling device, for example for an electronic device.
In cooling devices for use, for example in an electronic device in vehicles such as a passenger vehicle or industrial vehicle, a metal substrate on which a semiconductor device is to be mounted, an insulation plate, a metal layer for stress relief, and a heat sink are laminated so as to improve heat radiating performance. Generally, the metal substrate, the insulation plate, the metal layer, the heat sink are bonded together by means such as soldering or brazing.
Japanese Unexamined Patent Application Publication No. 2009-65144 discloses an electronic device, or a power module having a circuit board on one surface of which a semiconductor chip is mounted and on the other surface of which a heat sink is bonded. The circuit board is composed of a nitride-based ceramic plate, a circuit layer formed by a single-layer aluminum and laminated on one surface of the ceramic plate, and a metal layer for heat radiation formed by a double-layer aluminum with different purities and laminated on the other surface of the ceramic plate.
Specifically, aluminum-silicon-based brazing metal foils are provided between the circuit layer and the ceramic plate and between the ceramic plate and the metal layer. The circuit board is manufactured in such a way that the circuit layer, the brazing metal foil, the ceramic plate, the brazing metal foil and the metal layer laminated in this order are heated in vacuum atmosphere while being pressed so that the brazing metal foils are melted. After the manufacturing of the circuit board, the semiconductor chip is bonded to the circuit layer by soldering, and the heat sink is bonded to the metal layer by soldering or brazing. The semiconductor module is thus manufactured.
There has been a known problem with solder bonding that deterioration of the solder due to the use in a temperature changing environment over the years results in reduced heat transfer performance. To prevent this, the circuit board disclosed in the publication No. 2009-65144 is manufactured by brazing using brazing metal foil.
In the electronic device disclosed in the publication No. 2009-65144, however, the heat sink need to be bonded to the circuit board by soldering using solder the melting point of which is lower than that of the brazing metal foil. This is because bonding of the circuit board and the heat sink by brazing results in melting of the brazing metals for bonding of the components of the circuit board thereby to cause misalignment or deformation of such components, which makes it difficult to form the circuit board in desired shape. Thus, the circuit board and the heat sink are bonded generally by soldering which can be done at a lower temperature than brazing.
As described above, in the conventional cooling device as disclosed in the publication No. 2009-65144, the heat sink need to be bonded to the circuit board by soldering after the manufacturing of the circuit board, resulting in inefficient manufacturing and a decrease of heat radiating performance due to the use over the years.
The present invention is directed to providing a cooling device that allows efficient manufacturing and prevents decrease of heat radiating performance.